High differential pressure electrochemical cell

ABSTRACT

An electrochemical cell capable of operating in pressure differentials exceeding about 2,000 psi, using a porous electrode. The porous electrode comprises a catalyst adsorbed on or in a porous support that is disposed in intimate contact and fluid communication with the electrolyte membrane.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date of U.S.Provisional Application Serial No. 60/166,135, filed on Nov. 18, 1999,which is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to an electrochemical cell, and especiallyrelates to an electrochemical cell capable of operating at highdifferential pressure.

BACKGROUND OF THE INVENTION

Electrochemical cells are energy conversion devices, usually classifiedas either electrolysis cells or fuel cells. A proton exchange membraneelectrolysis cell functions as a hydrogen generator by electrolyticallydecomposing water to produce hydrogen and oxygen gases, and functions asa fuel cell by electrochemically reacting hydrogen with oxygen togenerate electricity.

Referring to FIG. 1, a partial section of a typical fuel cell 10 isdetailed. In fuel cell 10, hydrogen gas 12 and reactant water 14 areintroduced to a hydrogen electrode (anode) 16, while oxygen gas 18 isintroduced to an oxygen electrode (cathode) 20. The hydrogen gas 12 forfuel cell operation can originate from a pure hydrogen source, methanolor other hydrogen source. Hydrogen gas electrochemically reacts at anode16 to produce hydrogen ions (protons) and electrons, wherein theelectrons flow from anode 16 through an electrically connected externalload 21, and the protons migrate through a membrane 22 to cathode 20. Atcathode 20, the protons and electrons react with the oxygen gas to formresultant water 14′, which additionally includes any reactant water 14dragged through membrane 22 to cathode 20. The electrical potentialacross anode 16 and cathode 20 can be exploited to power an externalload.

The same configuration as is depicted in FIG. 1 for a fuel cell isconventionally employed for electrolysis cells. In a typical anode feedwater electrolysis cell (not shown), process water is fed into a cell onthe side of the oxygen electrode (in an electrolysis cell, the anode) toform oxygen gas, electrons, and protons. The electrolytic reaction isfacilitated by the positive terminal of a power source electricallyconnected to the anode and the negative terminal of the power sourceconnected to a hydrogen electrode (in an electrolysis cell, thecathode). The oxygen gas and a portion of the process water exit thecell, while protons and water migrate across the proton exchangemembrane to the cathode where hydrogen gas is formed. In a cathode feedelectrolysis cell (not shown), process water is fed on the hydrogenelectrode, and a portion of the water migrates from the cathode acrossthe membrane to the anode where protons and oxygen gas are formed. Aportion of the process water exits the cell at the cathode side withoutpassing through the membrane. The protons migrate across the membrane tothe cathode where hydrogen gas is formed.

In certain arrangements, the electrochemical cells can be employed toboth convert electricity into hydrogen, and hydrogen back intoelectricity as needed. Such systems are commonly referred to asregenerative fuel cell systems.

The typical electrochemical cell includes a number of individual cellsarranged in a stack, with the working fluid directed through the cellsvia input and output conduits formed within the stack structure. Thecells within the stack are sequentially arranged, each including acathode, a proton exchange membrane, and an anode. The anode, cathode,or both are conventionally gas diffusion electrodes that facilitate gasdiffusion to the membrane. Each cathode/membrane/anode assembly(hereinafter “membrane electrode assembly”, or “MEA”) is typicallysupported on both sides by support members such as screen packs orbipolar plates, forming flow fields. Since a differential pressure oftenexists in the cells, compression pads or other compression means areoften employed to maintain uniform compression in the cell active area,i.e., the electrodes, thereby maintaining intimate contact between flowfields and cell electrodes over long time periods.

In addition to providing mechanical support for the MEA, flow fieldssuch as screen packs and bipolar plates preferably facilitate fluidmovement and membrane hydration. These porous support members can alsoserve as gas diffusion media to effectuate proper transport of theoxygen and hydrogen gas. Increasing the rates of transport anduniformity of distribution of the fluids (including liquid water andoxygen and hydrogen gases) throughout the electrochemical cellsincreases operating efficiencies.

With the support of the screen packs, conventional electrochemical cellscan operate at a pressure differential of up to about 400 pounds persquare inch (psi). While suitable for their intended purposes, suchsupport members require additional manufacturing materials and steps,and may not be effective for cells operating at differential pressuresgreater than about 400 psi. In an electrolysis cell, for example, it isdesirable to operate the cell at about 1,000 psi or greater. Atpressures exceeding about 400 psi, and especially exceeding 600 psi,physical limitations of screen structures, i.e., the requirement of verysmall screen openings, hinders fluid transport therethrough, and thuslimits their usefulness.

In order to enable operation at pressures up to about 2,000 psi, porousplate technology has accordingly been developed. An exemplary porousplate is disclosed in U.S. Pat. Nos. 5,296,109 and 5,372,689, issued toCarlson et al. in 1994. As shown in FIG. 2, a porous sheet 213 isdisposed between the anode electrode 211 and the flow field (screen pack203) to provide additional structural integrity to the membrane 209.According to Carlson, porous sheet 213 further enables dual-directionalwater and oxygen flow.

Porous plate have also been previously disclosed in a paper presented atThe Case Western Symposium on “Membranes and Ionic and ElectronicConducting Polymer”, Cleveland, Ohio May 17-19, 1982. Again asillustrated in FIG. 2, this paper discloses that in order to prevent themembrane and electrode assembly from deforming into the flow fields, aporous, rigid support sheet 213 is inserted between the electrode 211and the flow field distribution component 203. The particulararrangement described employs a porous titanium support sheet on theanode electrode, and a carbon fiber paper, porous, rigid support sheeton the cathode electrode (p. 14). At page 2, this paper claims that suchcells were capable of operating at differential pressures ranging up togreater than 500 psi.

Use of porous plates are also disclosed in “Solid Polymer ElectrolyteWater Electrolysis Technology Development for Large-Scale HydrogenProduction”, Final Report for the Period October 1977-November 1981 byGeneral Electric Company, NTIS Order Number DE82010876, which isdirected to solid polymer electrolyte water electrolysis technology.Certain electrolyzer arrangements using porous titanium plates aredescribed, and as shown in FIG. 2 include an anode electrode 211, ananode electrode flow field of molded grooves 203, a cathode electrode207, a cathode flow field of molded grooves 205, and ion exchangemembrane 209 disposed between and in intimate contact with anode 211 andcathode 207. A porous sheet 213 is shown supporting ion exchangemembrane 209 and interposed between anode flow field 203 and anodeelectrode 211. It is stated on page 10 of the report that earlierdevelopment had shown that a support for the membrane and electrodeassembly was required on both the anode and cathode side to preventcreep of the membrane into flow areas. The anode support (porous sheet213) comprised a thin, titanium foil with many small holes etchedthrough for transport of water to the catalyst from the flow field andoutflow of the oxygen gas. To provide improved water flow rates, thesethin foil anode supports were reported to be replaced with poroustitanium plates (p. 66).

A significant disadvantage of porous plate technology is the additionalmaterials, manufacturing, and assembly expense that this element adds tothe cell assembly. What is accordingly needed in the art is a costeffective electrochemical cell capable of operating at high pressures,e.g. exceeding about 1,000 psi.

SUMMARY OF THE INVENTION

The above-described drawbacks and disadvantages are alleviated by anelectrochemical cell comprising a porous electrode, another electrode,and a membrane disposed between said electrodes; the method for usingthe electrochemical cell; the methods for making a porous electrode; andthe method for producing electrical power.

The electrochemical cell comprises: a first, porous electrode, a secondelectrode, and a membrane disposed therebetween, wherein said first,porous electrode comprises a catalyst disposed in physical contact withan electrically conductive, porous support; a flow field in fluidcommunication with said second electrode; a first fluid port in fluidcommunication with said first electrode; and a second fluid port influid communication with said second electrode. The first, porouselectrode accordingly comprises a porous catalytic structure whichprovides structural support for and integrity to the catalyst, reactivesites for the electrolysis of water to hydrogen and oxygen, a fluid flowfield for the working gases and fluids, and support for the membrane.

One method for using the electrochemical cell comprises: introducingwater to an oxygen electrode, wherein said oxygen electrode comprises acatalyst disposed in physical contact with an electrically conductiveporous electrode; dissociating the water to form hydrogen ions, oxygen,and electrons; moving said electrons through an external load to ahydrogen electrode; migrating said hydrogen ions through a electrolytemembrane to the hydrogen electrode; and producing hydrogen gas at saidhydrogen electrode.

One method for making the porous electrode comprises: sintering a layerof electrically conductive material to form a sintered, porous support;imbibing said sintered porous support with a solution of catalyst andsolvent; and removing the solvent to form the porous electrode, whereinsaid porous electrode preferably has a porosity greater than about 20%by volume.

An alternative method for making the porous electrode comprises: coatingan electrically conductive material with a solution of catalyst andsolvent; forming a layer of said coated electrically conductivematerial; optionally removing the solvent from said layer; and sinteringsaid layer to form the porous electrode, wherein said porous electrodepreferably has a porosity greater than about 20% by volume.

Still another method for making the porous electrode comprises: coatingan electrically conductive, porous support with a solution of catalystprecursor and solvent; and converting said catalyst precursor to acatalyst, wherein said porous electrode has a porosity preferablygreater than about 20% by volume.

The method for producing electrical power comprises: producing firstelectricity; introducing at least a portion of said first electricity toan electrochemical cell having an oxygen electrode, a hydrogenelectrode, an electrolyte membrane disposed therebetween, and anelectrical load in electrical communication with said oxygen electrodeand said hydrogen electrode, said oxygen electrode comprising a catalystdisposed in an electrically conductive porous flow field; introducingwater to said oxygen electrode; dissociating the water to form hydrogenions, electrons, and oxygen; migrating said hydrogen ions through saidelectrolyte membrane to said hydrogen electrode; moving said electronsthrough said electrical load to said hydrogen electrode; producinghydrogen gas at said hydrogen electrode; and using said hydrogen gas toproduce additional electricity when said first electricity is notavailable or is insufficient.

The above discussed and other features and advantages of the presentinvention will be appreciated and understood by those skilled in the artfrom the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, which are meant to be exemplary and notlimiting, and wherein like elements are numbered alike in the severalFigures:

FIG. 1 is a schematic diagram of a prior art electrochemical cellshowing a typical fuel cell reaction;

FIG. 2 is a cross-sectional view of a prior art electrochemical cellhaving a porous plate;

FIG. 3 is a cross-sectional view of one embodiment of theelectrochemical cell of the present invention; and

FIG. 4 is a graph of voltage measured over time of an electrolysis cellstack of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Although described in relation to a proton exchange membraneelectrochemical cell employing hydrogen, oxygen, and water, it isreadily understood that this invention can be employed with all types ofelectrochemical cells utilizing solid electrolytes, such as solid oxide,proton exchange membrane, and various reactants, including, but notlimited to, hydrogen, bromine, chlorine, oxygen, iodine, fluorine,methanol, and other fluids. Upon the application of different reactantsand/or a different electrochemical cell stack, the flows, reactions, andpreferred materials (e.g. catalysts and type of membrane) are understoodto change accordingly, as is commonly understood in relation to thatparticular type of electrochemical cell.

This electrochemical cell, cell stack, electrode, and a method foroperating the same enables high pressure differential operation, i.e.,operation at differential pressures exceeding about 1,000 psi,preferably exceeding about 2,000 psi, and most preferably up to orexceeding about 4,000 psi. The electrochemical cell stack is comprisedof at least one, and preferably a plurality of electrochemical cells.Each cell comprises an anode electrode and cathode electrode with anelectrolyte membrane disposed therebetween, and fluid flow fields influid communication with the electrodes. At least one of the electrodes,preferably the oxygen electrode (anode) in an electrolysis cell,comprises a porous catalytic structure which provides structural supportfor and integrity to the catalyst, reactive sites for the electrolysisof water to hydrogen and oxygen, and a fluid flow field for, e.g., thewater and oxygen.

The electrolyte membrane for both electrolysis and fuel cells can be ofany material typically employed for forming the membrane inelectrochemical cells. The electrolytes are preferably solids or gelsunder the operating conditions of the electrochemical cell. Usefulmaterials include proton conducting ionomers and ion exchange resins.Useful proton conducting ionomers can be complexes of an alkali metal,alkali earth metal salt, or a protonic acid with one or more polarpolymers such as a polyether, polyester, or polyimide, or complexes ofan alkali metal, alkali earth metal salt, or a protonic acid with anetwork or crosslinked polymer containing the above polar polymer as asegment. Useful polyethers include polyoxyalkylenes, such aspolyethylene glycol, polyethylene glycol monoether, polyethylene glycoldiether, polypropylene glycol, polypropylene glycol monoether, andpolypropylene glycol diether; copolymers of at least one of thesepolyethers, such as poly(oxyethylene-co-oxypropylene)glycol,poly(oxyethylene-co-oxypropylene)glycol monoether, andpoly(oxyethylene-co-oxypropylene)glycol diether; condensation productsof ethylenediamine with the above polyoxyalkylenes; esters, such asphosphoric acid esters, aliphatic carboxylic acid esters or aromaticcarboxylic acid esters of the above polyoxyalkylenes. Copolymers of,e.g., polyethylene glycol with dialkylsiloxanes, polyethylene glycolwith maleic anhydride, or polyethylene glycol monoethyl ether withmethacrylic acid are known in the art to exhibit sufficient ionicconductivity to be useful. Useful complex-forming reagents can includealkali metal salts, alkali metal earth salts, and protonic acids andprotonic acid salts. Counterions useful in the above salts can behalogen ion, perchloric ion, thiocyanate ion, trifluoromethane sulfonicion, borofluoric ion, and the like. Representative examples of suchsalts include, but are not limited to, lithium fluoride, sodium iodide,lithium iodide, lithium perchlorate, sodium thiocyanate, lithiumtrifluoromethane sulfonate, lithium borofluoride, lithiumhexafluorophosphate, phosphoric acid, sulfuric acid, trifluoromethanesulfonic acid, tetrafluoroethylene sulfonic acid, hexafluorobutanesulfonic acid, and the like.

Ion-exchange resins useful as proton conducting materials includehydrocarbon- and fluorocarbon-type resins. Hydrocarbon-type ion-exchangeresins can include phenolic or sulfonic acid-type resins; condensationresins such as phenol-formaldehyde, polystyrene, styrene-divinyl benzenecopolymers, styrene-butadiene copolymers,styrene-divinylbenzene-vinylchloride terpolymers, and the like, that areimbued with cation-exchange ability by sulfonation, or are imbued withanion-exchange ability by chloromethylation followed by conversion tothe corresponding quaternary amine.

Fluorocarbon-type ion-exchange resins can include hydrates of atetrafluoroethylene-perfluorosulfonyl ethoxyvinyl ether ortetrafluoroethylene-hydroxylated (perfluoro vinyl ether) copolymers.When oxidation and/or acid resistance is desirable, for instance, at thecathode of a fuel cell, fluorocarbon-type resins having sulfonic,carboxylic and/or phosphoric acid functionality are preferred.Fluorocarbon-type resins typically exhibit excellent resistance tooxidation by halogen, strong acids and bases. One family offluorocarbon-type resins having sulfonic acid group functionality is theNAFION® resins available from E. I. DuPont de Nemours Inc., Wilmington,Del.

The membrane is disposed adjacent to and in fluid communication with theporous electrode, which comprises a catalyst adsorbed on, and preferablythroughout a porous support. Possible catalysts include electrodecatalysts conventionally utilized in electrochemical cell systems, suchas platinum, palladium, rhodium, iridium, ruthenium, osmium, carbon,gold, tantalum, tin, indium, nickel, tungsten, manganese, and the like,as well as mixtures, oxides, alloys, and combinations comprising atleast one of the foregoing catalysts. Additional possible catalysts,which can be used alone or in combination with the above, includegraphite and organometallics, such as pthalocyanines and porphyrins, andcombinations comprising at least one of the foregoing catalysts, and thelike. Suitable catalysts are also disclosed in U.S. Pat. Nos. 3,992,271,4,039,409, 4,209,591, 4,272,352, 4,707,229, and 4,457,824, which areincorporated herein by reference in their entirety.

The catalyst can be in the form of discrete catalyst particles, and mayfurther comprise hydrated ionomer solids, fluorocarbons, other bindermaterials, other materials conventionally utilized with electrochemicalcell catalysts, and combinations comprising at least one of theforegoing catalysts. The ionomer solids can be any swollen (i.e.,partially disassociated polymeric material) proton and water conductingmaterial. Possible ionomer solids include those having a hydrocarbonbackbone, and perfluoroionomers, such as perfluorosulfonate ionomers(which have a fluorocarbon backbone). Ionomer solids and catalyststherewith are further described in U.S. Pat. No. 5,470,448 to Molter etal., which is incorporated herein by reference in its entirety.

The catalyst is adsorbed on and/or within a porous support as furtherdescribed below. The porous support can comprise any electricallyconductive material compatible with the electrochemical cell environment(for example, the desired pressure differential, preferably up to orexceeding about 4,000 psi, temperatures up to about 250° C., andexposure to hydrogen, oxygen, and water). Some possible materialsinclude carbon, nickel and nickel alloys (e.g., Hastelloy®, which iscommercially available from Haynes International, Kokomo, Ind.,Inconel®, which is commercially available from INCO Alloys InternationalInc., Huntington, W.Va., among others), cobalt and cobalt alloys (e.g.,MP35N®, which is commercially available from Maryland Specialty Wire,Inc., Rye, N.Y., Haynes 25, which is commercially available from HaynesInternational, Elgiloy®, which is commercially available from Elgiloy®Limited Partnership, Elgin, Ill., among others), titanium, zirconium,niobium, tungsten, carbon, hafnium, iron and iron alloys (e.g., steelssuch as stainless steel and the like), among others, and oxides,mixtures, and alloys comprising at least one of the foregoing materials.The particular form of the porous support, e.g., fibrous (random, woven,non-woven, chopped, continuous, and the like), granular, particulatepowder, preform, and the like are discussed in more detail below inconnection with manufacture of the porous electrode.

A number of methods may be used in the manufacture of the porouselectrode. For example, particulate materials of any geometry can beinfiltrated with the desired catalyst, e.g., in the form of an ink, viapainting, spraying, dipping, imbibing, vapor depositing, or the like.The infiltrated particles are then compacted using pressure, and thenvacuum sintered (e.g., co-sintered) to form the porous electrode. Theparticles before compaction may be either solid or porous.

Alternatively, porous support, e.g., in the form of a fibrous felt,woven or unwoven screen, a porous layer, or a combination comprising atleast one of the foregoing forms, or the like, is infiltrated with thecatalyst. Once the catalyst has been adsorbed onto at least somesurfaces of the porous support, any solvent is removed, leaving behindthe catalyst material. As used herein “adsorbed” is intended toencompass any adsorption onto a surface of the porous support (whetherthe surface is exterior or interior), as well as absorption within thematerial comprising the porous support, as may occur with certainconductive polymers, for example. The fibers or other forms maythemselves be solid or porous. Suitable porosities for the poroussupport are generally greater than about 10%, preferably greater thanabout 20%, and most preferably about 40 to about 90% by volume.

Still another method for making the porous electrode comprises coatingan electrically conductive, porous support with a solution of catalystprecursor and solvent; and converting said catalyst precursor to acatalyst.

In another embodiment, particulate or fibrous materials of virtually anygeometry are used to make a preform. This method accordingly comprisessintering a layer of electrically conductive material to form asintered, porous support; infiltrating the sintered porous support witha solution of catalyst and solvent; and removing the solvent to form theporous electrode. Suitable porosities for the porous support aregenerally greater than about 10%, preferably greater than about 20%, andmost preferably about 40 to about 90% by volume.

The porous electrode should have a porosity and pore size effective toenable migration of the appropriate fluid therethrough and dualdirectional fluid flow. Such porosities are readily determined by one ofordinary skill in the art, depending on the fluids, gases, pressures,and the like. Suitable porosities for the porous electrode are generallygreater than about 20%, preferably greater than about 40%, morepreferably about 20% to about 80%, and most preferably about 40% toabout 70% by volume. Typically a mean pore size of up to or exceedingabout 20 microns can be used, with up to about 15 microns preferred, andabout 2 to about 13 microns especially preferred.

Due to the three dimensional nature of the porous support, the electrodecan have a lower catalyst loading than conventional electrodes with asubstantially similar reactivity. For example, although catalystloadings exceeding about 10 milligrams per square centimeter (mg/cm²)can be used, loadings of less than about 2 mg/cm² are preferred, withloadings of about 0.01 mg/cm² to about 1 mg/cm² especially preferred. Incontrast, to obtain a similar reactivity, typical prior art electrodesrequired a catalyst loading of about 5 mg/cm² or greater.

The size and geometry of the porous electrode are dependent upon thespecific operating condition and application. For example, if the porouselectrode will replace the electrode and screen pack of a conventionalcell, the thickness of the porous electrode will be greater than in asystem which will also employ a screen pack. The porous electrodethickness is based upon whether a bipolar plate is employed, whether ascreen pack is employed, the opening size in the screen pack, pressureapplied across the membrane, operating conditions, material compositionand form (e.g., fiber (random, woven, non-woven, chopped, continuous,and the like), granular, particle, preform, powder, combinationcomprising at least one of the foregoing forms, and others), andporosity and strength of the porous electrode. Typically, for pressuresup to about 2,000 psi, a sintered particulate electrode and using a 3/0screen support, the porous electrode can have a thickness of up to about40 mil or more, with about 5 to about 20 mils more preferred, and about8 to about 12 mils especially preferred.

The second electrode can be a conventional electrode, e.g. a catalystlayer disposed in intimate contact with the membrane, or can be a secondsupported porous electrode as described above. The general compositionof the catalyst is preferably the same as described above in relation tothe porous electrode catalyst, wherein the catalysts employed on eachside of the membrane can be of substantially the same composition ordifferent compositions.

The flow fields of the electrochemical cell can comprise screen packs,can be formed by the porous electrode, bipolar plates with grooves orother flow features formed therein, or other type of support structure.Suitable screen packs comprise electrically conductive material, such aswoven metal, expanded metal, perforated or porous plates, fabrics (wovenand non-woven), ceramic (e.g., particulate filled ceramic), polymers orother material, or a combination thereof, which provide structuralintegrity to the membrane assembly while forming an appropriate flowfield for the various fluids and establishing an electron transport toand from the electrodes. Typically the screen packs are composed ofmaterial such as niobium, zirconium, tantalum, titanium, steels such asstainless steel, nickel, and cobalt, among others, as well as mixtures,oxides, and alloys comprising at least one of the foregoing materials.The geometry of the openings in the screens can range from ovals,circles and hexagons to diamonds and other elongated and multi-sidedshapes. The particular porous conductive material employed is dependentupon the particular operating conditions on that side of the membraneassembly. Examples of suitable screen packs are disclosed in commonlyassigned U.S. application Ser. No. 09/464,143, which is incorporatedherein by reference in its entirety.

In order to attain the desired pressures, in the electrochemical cells,it is preferable to at least dispose a porous electrode on the lowerpressure side of the membrane in conjunction with a screen pack.Although the porous electrode forms a suitable flow field and providessufficient structural integrity to the membrane up to pressures of about2,500 psi, screen packs can provide high flow throughput to enhanceremoval of process heat.

In order to allow transport of the electrons, the electrodeselectrically connect to an electrical load and/or power source. Theelectrical connection can comprise any conventional electrical connectorsuch as wires, a truss/buss rod, buss bars, combinations comprising atleast one of the foregoing connectors, or another electrical connector.

The hydrogen produced hereby can be stored as high-pressure gas, oralternatively, in a solid form, such as a metal hydride, a carbon basedstorage (e.g. particulates, nanofibers, nanotubes, or the like), orothers and combinations comprising at least one of the foregoing solidstorage forms.

FIG. 3 illustrates one embodiment of the electrochemical cell 301. Inthis embodiment, porous electrode 311 is disposed between and in bothfluid and electrical communication with hydrogen flow field 303 andmembrane 309. On the opposite side of membrane 309 is the oxygenelectrode 307 disposed between and in both fluid and electricalcommunication with the membrane 309 and the oxygen flow field 305.

In an alternative embodiment, the porous electrode could form the oxygenflow field and be disposed in fluid and electrical communication withthe membrane. Meanwhile, the hydrogen electrode would be in electricaland fluid communication with the hydrogen flow field and the oppositeside of the membrane.

The porous electrode can be employed on either or both sides of theelectrolyte, alone or in combination with a flow field, with variousother conventional electrochemical cell stack components being optionalbased upon the particular design chosen. Some possible optionalcomponents comprise protector lip(s), gasket(s), separator plate(s),pressure pad(s), spring washer(s), and the like.

The following examples are provided for the purpose of illustration anddo not limit the invention.

EXAMPLES Example 1

A porous oxygen electrode was formed by treating a 0.010 inch thicksintered titanium plate having approximately 50% porosity with acatalyst ink and binder comprising 3.7 weight percent (wt %) of anoxygen catalyst (as disclosed in expired U.S. Pat. No. 3,992,271, 50%Pt-50%Ir), 12.1 wt % of 5 wt % solution Nafion®, and 84.2 wt % deionizedwater. The porous plate was soaked in acetone then placed in the boilingink solution at 100° C. The acetone was vaporized in the solution whilethe catalyst and binder remained and imbibed into the porous titaniumcreating a porous electrode. The porous electrode was then heated to180° C. in an oven to activate the catalyst binder, thereby adsorbingthe catalyst to walls of the pores in the interior the porous electrode.This process was repeated until a catalyst loading of 0.8 mg/cm² wasachieved.

The electrode prepared in accordance with the above example was testedin an electrolysis cell stack loaded with a force of 63,200 lbs on anoverall cell area of 31.6 in² and configured in the following order:separator plate, screen pack (in accordance with U.S. application Ser.No. 09/464,143, the porous electrode, a Nafion® membrane (having aconventional catalyst disposed on the opposite side), a second screenpack (in accordance with U.S. application Ser. No. 09/464,143), a shim,a pressure pad (in accordance with U.S. patent application Ser. No.09/413,782), and another separator plate. A stainless steel ring wasfitted around the cell frames to provide lateral strength forhigh-pressure operation. This cell was hydrostatically tested to 2,500pounds per square inch gauge (psig) without overboard or crossover cellleakage.

Operational testing was conducted using the above-describedconfiguration at 2000 psi hydrogen, essentially atmospheric oxygenpressure, a temperature corrected to 50° C. (variation over the courseof testing was ±10° C.), a current density of 1000 Amps/cm². The waterwas passed on the oxygen side of the cell. As the data shown in FIG. 4illustrates, the cell shows a voltage degradation of less than 1microvolt per cell hour, indicating that the inventive porous electrodestructure is providing a combination of effective oxygen and water flow,as well as support.

Example 2

A porous oxygen electrode was formed by dipping a 0.03 square foot(27.87 cm²) porous titanium plate into 20 mL of an aqueous 6M HClsolution comprising 1 gram of iridium trichloride and 1 gram ofhexachloroplatinic acid that had been stirred for at least one hour.After dipping, the coated plate was dried for about 20 minutes in anoven (151° F., 61° C.). The dipping/drying cycle was performed fivetimes, then the dried, coated plate was heated to 500° C. for about 30minutes, to convert the chloride to the corresponding metal oxide(s).

The metal oxides are then reduced to the metal by chemical (e.g., boronhydride) or other means. Thus, the heated plate was then immersed in a3M aqueous solution of sulfuric acid and connected to a the negativeterminal of a power supply, making it the cathode, and a platinizedtitanium counterelectrode was connected to the positive terminal of thepower supply, making it the anode. The current was maintained at 2 voltsfor about 15 minutes, and then at 5 volts for about 15, resulting in thereduction of the metal oxides to the corresponding metals. At 1.5 volts,hydrogen appeared to evolve at the porous electrode (cathode).

The electrochemical cells and method of use thereof employing theabove-described electrodes have a number of advantageous features. Forexample, because hydrogen can be stored at high pressures (e.g., greaterthan about 400 psi), the electrode and the electrochemical cell stackcan be particularly useful in numerous applications such as to power avehicle, as a supplemental power source, or to convert intermittentpower into steady power. When powering a vehicle, the electrochemicalcell can be in fluid and/or electrical communication with the vehicle,e.g., with an internal combustion engine, a fuel cell, and/or canoperate as a regenerative fuel cell. For example, an electrolysis cellstack can produce hydrogen, which is stored, supplied to a vehicle, andthen used in the vehicle to produce motive power.

Intermittent power can be supplied in numerous manners: photovoltaiccells, intermittent due to night (non-solar) exposure; wind devices(e.g., windmills) intermittent when wind speed is low or substantiallynon-existent; hydroelectric power (e.g., water, typically dammed,directed through turbines), intermittent during drought conditions; gridpower, intermittent during power outages (e.g., lines are down due to astorm); combinations thereof, and the like. The electrochemical cellstack can be incorporated into a grid with one or more of the powersources to produce and store hydrogen while these intermittent powersources provide electricity. When the intermittent power sources are notavailable or additional electrical power is desired (e.g., during peakusage times), the stored hydrogen can be used to produce electricity.

The electrochemical cell employs at least one porous electrode whichenables high pressure differential operation, e.g., pressures up to andexceeding about 2,000 psi, with pressures up to about 6,000 psipreferred, and pressures up to and exceeding 10,000 psi possible. Suchhigh-pressure operation enables the direct filling of high-pressurehydrogen and/or oxygen tanks (cylinders or the like), without the use ofadditional equipment such as compressors and the like. This efficient,effective utilization of an electrochemical cell renders this technologyparticularly useful in areas requiring high pressure output with reducedsize, e.g. automotive industry, remote location fuel and/or electricitygeneration (such as undeveloped, inaccessible locations), and the like.For example, an electrochemical cell having at least one porouselectrode could be employed in or with a vehicle to recharge the vehiclewith hydrogen to use in producing electricity for motive power.

A further advantage is the improved efficiency of the catalyst. In priorart electrodes, the electrode was very thin (typically about less thanabout 0.1 mils) and relatively impermeable to the reactant.Consequently, reactions substantially only occurred on the electrodesurface. As a result, there was a limited reaction zone and a largepercentage of the catalyst (i.e., the sub-surface catalyst) remainedunused. In contrast, the porous electrode is three-dimensional andreactant permeable. Consequently, the catalyst has access tosubstantially all of the catalyst. Due to the high efficiency of thisthree-dimensional structure, the catalyst loading necessary to attainsubstantially similar catalyst activity as the prior art issubstantially reduced, e.g., by about 3 orders of magnitude (e.g., about10 mg/cm², conventionally, to as low as about 0.01 mg/cm² with theporous electrode). Due to the structural integrity provided by thisporous electrode, it can be used alone or in conjunction with anadditional fluid flow field(s) (e.g., one or more screen packs or plateshaving flow channel features).

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustrations and not limitation.

What is claimed is:
 1. An electrochemical cell, comprising: a firstelectrode, a second electrode, and a membrane disposed therebetween,wherein said first electrode comprises an electrically conductive,sintered, porous support comprising an infiltrated catalyst with acatalyst loading of about 0.01 mg/cm² to about 10 mg/cm²; a flow fieldin fluid communication with said second electrode; a first fluid port influid communication with said first electrode; and a second fluid portin fluid communication with said second electrode.
 2. An electrochemicalcell as in claim 1, wherein said second electrode or second electrodeand flow field are a second, porous electrode comprising a catalystadsorbed onto a surface of an electrically conductive, sintered, poroussupport.
 3. An electrochemical cell as in claim 1, further comprising anadditional flow field disposed in fluid and electrical communicationwith said first electrode.
 4. An electrochemical cell as in claim 1,wherein the electrochemical cell is a fuel cell, an electrolysis cell,or a regenerative fuel cell.
 5. An electrochemical cell as in claim 1,wherein the electrochemical cell is capable of operating at differentialpressures across the membrane up to and exceeding about 4000 psi. 6.electrochemical cell as in claim 1, wherein said first electrode has aporosity of about 20% to about 80% by volume.
 7. An electrochemical cellas in claim 1, wherein said first electrode has a porosity of about 40%to about 70%.
 8. An electrochemical cell as in claim 1, wherein saidfirst electrode has a thickness of up to about 40 mils.
 9. Anelectrochemical cell as in claim 8, wherein said first electrode has athickness of about 5 mils to about 20 mils.
 10. An electrochemical cellas in claim 9, wherein said first electrode has a thickness of about 8mils to about 12 mils.
 11. An electrochemical cell as in claim 1,wherein the electrochemical cell is in fluid and/or electricalcommunication with a vehicle.
 12. An electrochemical cell as in claim 1,wherein said porous support further comprises nickel, cobalt, titanium,zirconium, hafnium, niobium, tungsten, carbon, iron, of a mixture, or analloy thereof.
 13. An electrochemical cell as in claim 1, wherein saidcatalyst comprises platinum, palladium, rhodium, iridium, ruthenium,osmium, carbon, gold, tantalum, tin, indium, nickel, tungsten,manganese, graphite, organometallics, mixtures, oxides, or alloysthereof.
 14. An electrochemical cell as in claim 1, wherein saidcatalyst further comprises ionomer solids, fluorocarbons, or acombination thereof.
 15. An electrochemical cell as in claim 14, whereinsaid ionomer solid is a swollen proton and water conducting material.16. An electrochemical cell as in claim 14, wherein said ionomer solidsare ionomers having a hydrocarbon or fluorocarbon backbone.
 17. Anelectrochemical cell as in claim 1, wherein said porous supportcomprises fibers, screens, sintered particulates, or a combinationthereof.
 18. An electrochemical cell as in claim 1, wherein said firstelectrode has a mean pore size of about 2 to about 13 microns.
 19. Anelectrochemical cell, comprising: a first electrode, a second electrode,and a proton exchange membrane disposed therebetween, wherein said firstelectrode comprises an electrically conductive, sintered, porous supportcomprising an infiltrated catalyst with a catalyst loading of about 0.01mg/cm² to about 10 mg/cm²; a flow field in fluid communication with saidsecond electrode; a first fluid port in fluid communication with saidfirst electrode; and a second fluid port in fluid communication withsaid second electrode.